In general, it is preferable for display apparatuses such as liquid crystal display apparatuses (LCDs) that the display apparatuses have such a gradient characteristic that output brightness is liner with respect to an input gradation signal. Here, a characteristic represented by a relationship between an input gradation signal value and the output brightness is referred to as γ characteristic. The γ characteristic specifically refers to a proportional change of brightness L of a display apparatus with respect to γ-th power of an input gradation signal value E that is inputted to the display apparatus. Thus, the γ characteristic is represented by a formula: L=KEγ (γ=2.2 to 3 generally; K is constant).
As described above, in the most of the display apparatuses, the output brightness is not linear with respect to the input gradation signal, that is, the most of the display apparatuses have the γ characteristic. Thus, in the most of the display apparatuses, it is impossible to attain correct gradient display without special treatment.
Therefore, it is necessary to carry out correction in the display apparatuses in advance by using 1/γ power of the input gradation signal value E, which is shown in FIG. 7 as “reverse γ character of general image (E1/γ)”. Hereby, it is possible to perform correct gradient display as shown in FIG. 7 as “correct γ characteristic (EY)”. This correction is referred to as γ correction. By carrying out the γ correction, it is possible to attain the linear relationship of the output brightness with respect to the input gradation signal, that is, L=KE (K is constant), as the “correct γ characteristic (Eγ)”.
Incidentally, in order to attain the γ characteristic corresponding to the γ reverse characteristic, a γ correction circuit of the display apparatus as shown in FIG. 8 is used. The γ correction circuit realizes suitable sixty four gradations by using each selector to select one of sixty four output terminals, which are sectioned as sixty four stages. By doing this, the correct gradient display is attained. Note that the γ correction circuit is, as shown in FIG. 9, built in a source driver 71 of an LCD 70.
In the conventional LCD, it is impossible to change a most suitable value of the γ correction during operation of the LCD, after the most suitable value is once set. In case where a display mode is not switched over, there is no problem with this arrangement.
However, a semi-transmissive LCD is on the market recently. The semi-transmissive LCD takes advantages of both of the reflection method (referred to as “mode 1”) and the transmission method (referred to as “mode 2”). In the reflection method, outside light is used while a backlight is turned off. Thus, the reflection method is used in a bright place, for low power consumption. In a dark place, used is the transmission method in which the backlight is used as in a conventional method.
In the semi-transmissive LCD, the same image looks differently when the semi-transmissive LCD is switched over from the transmission method to the reflection method, because gradation-brightness characteristics of the transmission method and the reflection method are completely dissimilar to each other, as shown in FIG. 10. This is because applied voltage (V)-transmissivity (T) characteristics of the transmission method and the reflection method are dissimilar to each other due to differences between the transmission method and the reflection method in terms of the transmissivity and the applied voltage, as shown in FIG. 11. It should be noted that FIG. 11 only discuss on the applied voltage (V)-transmissivity (T) characteristics, but the same is true for the applied voltage (V)-reflectivity characteristics.
Moreover, there is a case where it is desirable that respective display modes perform different gradation displays, because, if there is only one gradation setting, a user may feel strange in the display.
In view of this, as shown in FIG. 12, it is so arranged that a plurality of characteristic curves of gradation-voltage application characteristic of a source driver are provided in advance and the characteristic curves to be used are switched over in accordance with the switchover of the display modes. In FIG. 12, there are only two settings. Of course, however, there may be provided more than two settings, so that one of the settings can be selected for each display mode. With this arrangement, as shown in FIG. 13, it is possible to attain complete matching of the gradation-brightness characteristics between the transmission method and the reflection method.
As a conventional display apparatus capable of changing gradation levels according to which display mode is used, for example, Japanese Publication for Unexamined Patent Application “Tokukai No. 2000-193936 (published on Jul. 14, 2000) discloses a display apparatus including two types of reference potential generating circuits 81 and 82, as shown in FIG. 14. By selecting one of the reference potential generating circuits 81 and 82 in accordance with reflection/transmission judging signal, voltage (V)-transmissivity (T) characteristics of the “transmission” and “reflection” are matched (become very similar) for each gradation, while the applied voltage (V)-reflectivity characteristics of the “transmission” and “reflection” are also matched (become very similar) for each gradation.
Moreover, in view of a problem that γ correction coefficients cannot be switched over according to which mode is used, because a circuit generating a reference voltage for a γ correction circuit has such an arrangement voltages of the circuit are determined in accordance with a voltage division ratio (a ratio of voltage division using resistors), Japanese Publication for Unexamined Patent Application “Tokukaihei No. 10-333648 (published on Dec. 18, 1998) and Japanese Publication for Unexamined Patent Application “Tokukaisho No. 63-38989 (published on Feb. 19, 1988) disclose arrangements in which information regarding a reference voltage of a γ correction circuit 90 is stored in a memory 91, so that reference voltages V1 to V10 are generated by retrieving the information and performing D/A (from digital to analog) conversion of the thus retrieved information. With this arrangement, it is possible to easily attain a γ correction coefficient arbitrarily.
However, in the conventional display apparatus disclosed in Japanese Publication for Unexamined Patent Application “Tokukai No. 2000-193936 (published on Jul. 14, 2000), it is impossible to reset γ correction values for the respective display modes once the γ correction values are once set. Thus, it is impossible to switch the γ correction values when the display modes are switched over, so that the display modes are performed with the most suitable γ correction values. Thus, it is a problem that the respective display mode cannot have very similar gradation-brightness characteristics with high accuracy.
Moreover, in Japanese Publication for Unexamined Patent Application “Tokukaihei No. 10-333648 (published on Dec. 18, 1998) and Japanese Publication for Unexamined Patent Application “Tokukaisho No. 63-38989 (published on Feb. 19, 1988), gradation correction voltages can be switched over only discretely. Thus, it is necessary to have a large number of input points of the gradation correction voltages for attaining smooth switchover of the gradation characteristics. As a result, this art also has the problem that the respective display mode cannot have very similar gradation-brightness characteristics with high accuracy.